Cermet alloy composition

ABSTRACT

A CERMET-TYPE ALLOY POSSESSING IMPROVED SULFIDATION RESISTANCE WHICH IS ADAPTABLE FOR FORMNG PROTECTIVE SURFACE COATINGS ON VARIOUS HIGH-TEMPERATURE, CORROSION-RESISTANT NICKEL, COBALT AND IRON BASE ALLOYS, AS WELL AS SHAPED PARTS OF THE ALLOY PER SE. THE INVENTION ALSO ENCOMPASSES A NOVEL METALLIC POWDER CONTAINING NICKEL AND/OR COBALT IN COMBINATION WITH CHROMIUM FOR FORMING THE CONTINUOUS MATRIX OF THE RESULTANT ALLOY, AS WELL AS TITANIUM AND/OR ZIRCONIUM AS THE REACTIVE METAL CONSTITUENTS WHICH ARE ADAPTED TO UNDERGO AN EXOTHERMIC REACTION WITH BORON UPON FUSION OF THE POWDER MIXTURE AT AN ELEVATED TEMPERATURE IN A SUBSTANTIALLY INERT ATMOSPHERE, THEREBY FORMING THE CORRESPONDING BORIDES OF THE REACTIVE METALS IN SITU, WHICH ARE SUBSEQUENTLY PRECIPITATED AS UNIFORMLY DISPERSED DISCONTINUOUS PHASES IN THE CONTINUOUS PHASE OF THE BASE ALLOY MATRIX. IN ADDITION, THE PARTICULATED MIXTURE FURTHER CONTAINS A CONTROLLED PROPORTION OF A FINELY-PARTICULATED REFRACTORY OXIDE COMPRISED OF ALUMINA WHICH SIMILARLY IS DISPERSED AS DISCRETE PHASES IN THE CONTINUOUS BASE ALLOY MATRIX AT THE COMPLETION OF THE EXOTHERMIC REACTION, FORMING A CERMET-TYPE ALLOY WHICH POSSESSES EXCEPTIONAL RESISTANCE TO SULFIDATION AND OXIDATION ATTACK AT ELEVATED TEMPERATURES.

Dec. 1.2, 1972 N. BREDZS CERMET ALLOY coMPosITIoN Filed Sept. 18, 1970 lllllllllllk 'United States Patent Oiiice 3,705,791 CERMET ALLOY COMPOSITION Nikolajs Bredzs, Detroit, Mich., assignor to Wall Colmonoy Corporation Filed Sept. 18, 1970, Ser. No. 73,460 Int. Cl. B32b 15/00 US. Cl. 29--195 I6 Claims ABSTRACT OF THE DISCLOSURE A cermettype alloy possessing improved sulfidation resistance which is adaptable for forming protective surface coatings on various high-temperature, corrosion-resistant nickel, cobalt and iron base alloys, as well as shaped parts of the alloy per se. The invention also encompasses a novel metallic powder containing nickel and/or cobalt in combination with chromium for forming the continuous matrix of the resultant alloy, as well as titanium and/ or zirconium as the reactive metal constituents which are adapted to undergo an exothermic reaction with boron upon fusion of the powder mixture at an elevated temperature in a substantially inert atmosphere, thereby forming the corresponding borides of the reactive metals in situ, which are subsequently precipitated as uniformly dispersed discontinuous phases in the continuous phase of the base alloy matrix. In addition, the particulated mixture further contains a controlled proportion of a iinelyaparticulated refractory oxide comprised of alumina which similarly is dispersed as discrete phases in the continuous base alloy matrix at the completion of the exothermic reaction, forming a cermet-type alloy which possesses exceptional resistance to suldation and oxidation attack at elevated temperatures.

BACKGROUND OF THE INVENTION Various alloy compositions and protective coating conipositions have heretofore been used or proposed for use in connection with high-temperature operations which possess the necessary resistance to oxidation, thermal fatigue and sufidation so as to provide satisfactory performance over prolonged operating periods. Such heat-resistant alloys of the general types heretofore used include various types of stainless steels, nickel-base alloys, as well as the so-called superalloys including Hastelloy-X, Inconel 600 and the like. It has also been proposed to apply various protective coatings to the foregoing high-temperature oxidation-resistant alloys so as to provide still further improvements in their high-temperature performance and durability. While such alloys and protective coatings for heat-resistant alloys have provided substantial improvements in their high-temperature performance, a continuing problem has been the susceptibility of such metal alloys to suldation attack at elevated temperatures. The problem of suldation attack of such heat-resistant metal alloys is particularly pronounced in aircraft gas turbines in which the sulfur constituent in jet fuel, upon combustion in the presence of sodium chloride from sea air, produces a slag containing sodium sulfate, which effects a rapid deterioration of such alloys as the result of the formation of low-melting eutectics at their grain boundaries.

In accordance with the present invention, an improved cermet-type alloy composition is provided which possesses unexpectedly good sulidation resistance and, therefore, can be employed for the fabrication of parts or components which are to be exposed to high-temperature sulfurcontaining environments or, alternatively, can be used as protective coatings on substrates composed of conventional heat-resistant alloys to effect an improvement in their sulfidation resistance. The present invention also pro- 3,705,791 Patented Dec. l2, i972 vides a simple, economical and reproducible method for making the suliidation-resistant alloy, as well as coatings of the alloy on various metal substrates, and is further directed to a fnely-particulated mixture of the various constituents which, upon exotherinic reaction, produce the alloy or protective alloy coating of the present invention.

SUMMARY OF THE lINVENTION The benefits and advantages of the present invention are achieved by providing an improved cermet-type alloy either in the form of an ingot or solid part comprised of the alloy, or in the form of a thin protective coating on a metal substrate. In its composition aspects, the cermettype alloy comprising the present invention is broadly composed of a substantially continuous phase or matrix of a nickel and/ or cobalt base alloy which may additionally contain chromium in amounts of from about 10% to about 60%, in combination with other conventional impurities and other alloying agents of the types and in the amounts which do not appreciably reduce the heat-resistant properties of the continuous alloy matrix. Distributed substantially uniformly throughout the continuous phase is a discontinuous phase comprised of precipitated borides of titanium and/or zirconium which are present in the form of compressed crystals and are present in an amount ranging from about 2% up to about 40% by weight of the alloy. lIn addition, the discontinuous phase further comprises fne-sized particles of aluminum oxide (A1203) which are present in amounts up to about 8% by Weight and preferably are controlled in amounts of from about 5% up to about 7% by Weight. The surface of the alloy is further characterized as having .a glaze-type surface iinish which is believed to contribute: to its suldation resistance.

In accordance with the method aspects of the present invention, the cermet-type alloy is formed by exothermically reacting a metallic powder containing the matrix metals, the refractory aluminum loxide, the reactive titanium and/ or zirconium metal andthe reactive nonmetallic boron constituent which are present in controlled amounts so as to effect the formation of the corresponding titanium and/ or zirconium boride in situ as a result of an exothermic reaction in response to heating the metallic powder mixture to an elevated temperature at which at least a partial fusion thereof occurs. In accordance with this method, the elevated temperature to which the powder mixture is heated during the fusion process and as a result of the heat liberated during the exothermic reaction, effects the formation of a continuous metallic matrix in which the zirconium and/or titanium borides are substantially uniformly distributed along with the refractory oxide particles, providing a protective coating and/or alloy coniponent which has been found to possess unexpected sulfidation resistance. In forming the ingot and the protective coating, the powder mixture is fused at an elevated temperature which usually ranges from about 19i00 F. to about 2200 F. in the presence of a substantially inert atmosphere. In the formation of an ingot or pre-shaped part, the metallic powder mixture can be placed in a suitable refractory mold having the desired configuration, whereby the resultant fused alloy will assume the contiguration of the mold, requiring only minimal final finishing operations. In the formation of heat and suliidation resistant protective coatings, the particulated powder mixture is applied to the surface of a metal substrate in an amount so as to provide a resultant fused protective coating having an average thickness generally ranging from about 0.001 to about 0.010 inch, and preferably an average thickness of about 0.002 to about 0.005 inch.

In a further composition aspect of the present invention, a novel powder mixture is provided composed of the various constituents including the matrix metals, the

reactive metals, boron and the aluminum oxide which, upon application to a substrate, or fusion in a suitable mold, provides the requisite exothermic reaction to form the cermet-type alloy of the present invention. Such power mixtures are preferably controlled in particle size when used for forming protective coatings to assure coatings of substantially uniform thickness and composition throughout while when the powder mixture is utilized for forming parts or ingots of the resultant alloy, greater latitude is provided in the specific size and size distribution of the powder particles.

Further advantages and benefits of the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a magnified cross-sectional View of a substrate, such as a heat-resistant alloy, coated on one of its surfaces with a powder mixture temporarily bonded by a suitable organic binder so as to form an adherent coating;

FIG. 2 is a magnified fragmentary cross-sectional view of the same substrate shown in FIG. 1 after the particulated mixture has been fused and exothermically reacted at an elevated temperature;

FIG. 3 is a photomicrograph at an enlargement of 480 times of the metallurgical structure of a protective coating of a cermet-type alloy made in accordance with the practice of the present invention; and

FIG. 4 is a vertical sectional view, partly schematic, illustrating a furnace containing a refractory mold therein filled with the powder mixture prior to exothermic reaction thereof so as to form a part having an exterior configuration conforming to the contour of the mold cavity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It will be understood that the amounts of the various alloy constituents in the form of the initial powder mixture, as well as in the resultant fused alloys, are described in terms of percentages by weight in the specification and subjoined claims unless expressly indicated to the contrary.

In the formation of the cermet-type alloy, either in the form of a solid part or ingot, or in the form of a protective coating on a suitable metal substrate, a powder mixture is initially formed which contains the matrix metals, the reactive metals, boron and the refractory aluminum oxide particles. Specifically, the powder blend or mixture contains the metals adapted to form the continuous phase or matrix of the resultant alloy which essentially consists of nickel and/ or cobalt in addition to chromium, as well as further minor amounts of other alloying agents and conventional impurities which can ibe tolerated in amounts that do not appreciably detract from the high-temperature oxidation and sulfidation resistance and mechanical properties of the resultant matrix metals. In accordance with the present invention, the matrix metals comprise from about 50% up to about 97%, and preferably, from about 60% to about 90% of the total powder mixture. Based on the total of the matrix metals present, the chromium constituent comprises from about to about 60%, and preferably, from about to about 35% thereof. While the cobalt constituent can be substituted in whole or in part for the nickel constituent in the matrix metals, the use of nickel itself constitutes a preferred practice and wherein the cobalt constituent may be present in amounts up to about 5%. In addition to the three basic matrix metals, namely: nickel, combalt and chromium, the matrix metal mixture additionally may include hardening and/or strengthening agents so as to provide the desired physical properties of the resultant alloy consistent with its intended end use. Such alloying agents may typically include iron in amounts generally up to about 5%, manganese in amounts usually up to about 1%, as well as other conventional impurities such as aluminum, carbon, etc.

In addition to the matrix metals, the powder blend contains as a further essential constituent, at least one reactive metal which is selected from the group consisting of titanium, zirconium and mixtures thereof. Boron, as a nonmetalli-c reactive element, is another essential constituent which is effective during subsequent fusion of the powder mixture at an elevated temperature to exothermically react with the reactive metal forming the corresponding boride. It is conventionally preferred that the boron constituent be present in a stoichiometric proportion relative to the reactive metal present such that substantially all of the boron in the final cermet-type alloy is in the combined state. The titanium and zirconium reaction metals, or mixtures thereof, on the other hand, can be employed in excess amounts above the stoichiometric proportions so that the resultant alloy will contain residuary unreacted amounts of these two constituents.

The parti-cular quantity of the reactive metals and boron present in the powder blend is controlled so that the resultant alloy contains from about 2% to about 40% and preferably from about 4% to about 30% of the corresponding boride reaction compounds and/or complexes. In order to provide such residual reaction products, the powder blend itself is prepared so as to contain from about 1.5% to about 28% titanium or 1.8% to about 36% zirconium and from about 0.5% to about 12% boron. As previously indicated, the specific amount of boron present in the powder mixture is controlled within the aforementioned range in consideration of the particular quantity of the titanium and/or zirconium reactive metals present such that substantially all of the boron will enter into the exothermic reaction, thereby being present in the resultant alloy in a combined form. Preferably, the amounts of the reactive metals and boron are controlled so as to be in stoichiometric proportions.

In addition to the foregoing matrix metals and reactive constituents, the powder blend further contains aluminum oxide (A1203) present in a linely-particulated form. The alumina particles can be used in amounts up to about 8% by weight and preferably in amounts of from about 5% to about 7% by weight of the powder blendand/or of the resultant cermet alloy. While noticeable improvement occurs in the sulfidation resistance of cermet-type alloys incorporating as little as about 1% of the aluminum oxide, it is usually preferred to include higher amounts such as within the aforementioned preferred range in which maximum benefits are obtained. Usually, amounts of the aluminum oxide in excess of about 8% cannot be used since a two-phase mixture comprising the cermet-type alloy and a portion of the aluminum oxide in a free-flowing powdery state is obtained after the fusion and exothermic reaction of the powder blend.

The aluminum oxide is employed in a finely-particulated condition and preferably is of a particle size less than about 20 microns. While particle sizes of less than about 2 microns also can be satisfactorily employed, excessive dusting has sometimes" been encountered during the blending and application of the powder mixture and it is for this reason that average particle sizes above about 2 microns up to about 20 microns are preferably employed.

The powder blend containing the several constituents is conveniently Iprepared by mixing measured quantities of the individual components forming a substantially homogeneous powder blend. It is usually preferred to introduce the matrix metals and the reactive metals in the form of a prealloyed powder, which substantially simplies the handling of the reaction metals, which are susceptible to oxidation attack when in the pure elemental state. Further advantages are achieved when the prealloyed powder is controlled in proportion such that the constituents form or approach a lower melting point eutectic which reduces the threshold temperature to which the powder blend must be heated to effect a partial fusion and initiation of the exothermic reaction in forming the resultant cermet-type alloy. In accordance with this preferred practice, prealloyed powder containing 70% of titanium or zirconium and 30% nickel have been found to provide particular benefits, while prealloyed nickel powders containing 70% titanium and 30% nickel or, alternatively, 70% zirconium and 30% nickel, have also provided distinct benets. Similar eutectic prealloyed compositions when cobalt constitutes the base metal include prealloyed powders of 30% cobalt and 70% titanium and cobalt and 85% zirconium.

The metallic particles are also controlled in average particle size so as to preferably be within a range of from about microns up to about 500 microns. In the use of the powder blend for forming ingots or solid shaped articles, such as by fusion and exothermic reaction in a refractory `mold, greater latitude is provided in particle size enabling the use of metallic powder particles approaching the upper end of the aforementioned size range. On the other hand, when the powder blend is to be employed for forming protective coatings, the control of particle size is more important in order to achieve uniformity in the coating, as well as facilitating the application of the blend to the substrate. For this purpose, it has been found that a control of the particle size at the lower portion of the aforementioned range, that is, within a range of from about 20 microns up to about 150 microns, facilitates the attainment of satisfactory protective coatings. Generally, when the metallic particles of the powder blend are of a size less than about 20 microns, some inhibition is encountered in obtaining a substantially cornplete exothermic reaction between the reaction metal and the boron constituent. On the other hand, when the particle size of the powder blend is substantially greater than 150 microns, difficulty is encountered in some instances in effecting a uniform application of a powder blend to a substrate. In view of the foregoing, it is generally preferred to control the particle size of the metallic constituents of the powder blend to within a range of from about 2O microns to about 150 microns when employing the powder blend for protective coating applications and from about 20 microns up to about 500 microns when froming shaped ignots and parts. When utilizing the powder blend for forming protective coatings, the use of powder particles which are randomly distributed over substantially the entire usable size range provide for maximum coating densities and thereby improved quality and physical properties of theV resultant protective coatings formed.

In the practice of the method of the present invention, whereby the powder blend is employed for forming a protective coating on all or a selected portion of a substrate, the powder blend is applied by any of the means well known in the art in the form of a substantially uniform layer or, alternatively, in the form of a controlled non-uniform layer, as may be desired, to all or that portion of the surface of a substrate to be protected. In order to retain the powder particles on the surface of the substrate prior to the subsequent fusion operation, it is usually preferred to employ a suitable organic binder, effecting a temporary bond of the powder layer to the substrate. The binder is of a composition such that it will thermally decompose without leaving any residue during the subsequent fusion step. Binders of the various types known in the art which are satisfactory for use in the practice of this method include, for example, solutions of plastic materials such as polyethylene, polypropylene, polyvinyl, polyvinylidene, polyvinyl alcohol, acrylic resins, such as polymethylmethacrylate, or the like. The organic binder can be suitably mixed with the powder to provide a mixture of the desired viscosity such as a liquid slurry, paint or paste, that can be directly applied to the substrate to be protected, such as by spray application, dipping, brushing, flooding, or the like. Ordinarily, spray application, when feasible, constitutes the preferred practice in which the organic binder is formed as a separate spray into which the dry unheated powder particles are injected,

6 forming a composite spray that is directed against the surface of the object to be coated.. The admixture of the organic binder with 'the powder particles during their travel toward the substrate forms a substantially uniform lm which, upon drying, can be. readily handled prior to the fusion step.

The specic thickness of the coating applied can be varied so as to provide a resultant fused and reacted protective coating of the desired end thickness. Conventionally, it has been found that powder blends of a particle size and composition within the ranges as hereinabove described have a density whereby an initial temporarily bonded coating deposited in an amount of about milligrams (img.) per square inch produces a resultant fused coating of a thickness of about 0.001 inch. Employing the foregoing as a basic guideline, protective coatings averaging in thickness from about 0.001 to about 0.010 inch can be produced by employing multiples of the foregoing initial coating weight.

In another method aspect of the present invention, the powder blend can be employed for forming ingots or pre-shaped parts or components of the cermet-type alloy by placing the powder in a suitable mold having a cavity of the desired contour. The powder blend is preferably compacted such as by subjecting it to supersonic vibration to attain maximum packing density. Ordinarily, no organic binder is necessary when forming integral components of the cermet alloy employing mol-ds.

Regardless of whether an ingot, shaped part or protective coating of the cermet-type alloy is being formed, the powder blend is heated to an elevated temperature generally ranging from about 1900 F. up to about 2200" F., at which a partial fusion or melting of the metal particles occurs initiating the exothermic reaction. The maximum temperature to which the powder blend is heated is usually dictated by practical considerations such as the temperature limitations of the furnace used, and in the case of forming protective coatings, the particular properties of the base metal to which the layer of powder is applied. During the heating of the powder blend to the elevated temperature, and during the subsequent exothermic reaction, any organic binder used for retaining the powder particles together, such as in the case of protective coatings, thermally decomposes and volatilizes, leaving a residuary fused metal alloy substantially devoid of any residual binder contamination. To avoid oxidation attack of the powder particles during their heat-up to the fusion temperature and during the subsequent exothermic reaction, a non-oxidizing atmosphere is employed of which commercially attainable medium-high vacuums or, alternatively, substantially dry inert gases, such as commercially available helium or argon, have been found particularly satisfactory.

The nature of the specific exothermic reaction that takes place between the reactive metal or metals and the boron constituent present is not entirely lunderstood at the present time. It is believed however, that the reaction occurs so as to form a ceramic compound or complex of boron with titanium and/ or zirconium which is substantially insoluble in the nickel/ cobaltchromium matrix and which subsequently agglomerates to form precipitated discrete discontinuous phases dispersed substantially uniformly through the continuous phase. During the exothermic reaction, the aluminum oxide particles present are substantially uneffected but similarly are entrained and become uniformly dispersed throughout the continuous fused matrix. Stoichiometrically, the reaction of one gram atom titanium and two gram atoms of boron produces titanium diboride (TiB2), while the reaction of one gram atom of zirconium and two gram atoms of boron produces zirconium diboride (ZrBz). In addition to these specific chemical products, it is also believed that various complexes are produced between the reaction metals and the boron constituent during the course of the exothermic reaction.

It is also contemplated within the scope of the present invention that in addition to the titanium and zirconium boride ceramic compounds formed in situ during the exothermic reaction, the `ceramic constituent of the cermet-type alloy can be further supplemented by the direct addition of titanium nitride (TiN) to the initial powder mixture in proportions ranging up to about The inclusion of the titanium nitride has been found in some instances to further improve the oxidation resistance of the resultant alloy and is incorporated in the initial powder blend in a manner similar to the refractory oxide constituent.

In accordance with the foregoing description, the formation of a suitable protective coating on a metal substrate will now be described with reference to FIGS. 1 and 2 of the drawing. As illustrated in FIG. 1, a substrate, such as a sheet 6 preferably composed of a heat-resistant alloy, is provided with a coating 8 on one or on both faces surfaces thereof as may be desired. The specific coating comprises the individual metal particles and particles of the refractory oxide which are retained in the form of a substantially uniform layer by means of an organic binder. As previously indicated, the coating as applied can be restricted to only certain portions of the surface of the substrate and/or can be of controlled different thicknesses to provide the desired properties of the resultant article. The coated sheet 6, having the coating 8 thereon, is thereafter heated to an elevated temperature in an inert atmosphere such as a temperature from about 1900 F. to about 2200 F. in an argon atmosphere, effecting a fusion of the matrix metals and an exothermic reaction between the reaction metals and the boron constituent, accompanied by a thermal decomposition and volatilization of the organic binder. The resultant coated sheet, after cooling, is exemplified in FIG. 2 wherein the resultant coating 10' is tenaciously bonded to the sheet 6 through a diffusion zone indicated at 12 comprising an alloying of the coating metals with the substrate. The coating 10 is further characterized as incorporating a plurality of individual particles comprising discrete discontinuous phases of the alumina particles indicated at 14 and the boride reaction compounds indicated at 16. The particles 14 and 16 are substantially uniformly dispersed throughout the continuous phase of the nickel/cobalt-chromium matrix metal indicated at 18 in FIG. 2. The resultant fused coating is further characterized as incorporating a glazed surface finish which is schematically indicated at 20 in FIG. 2 which is formed during the exothermic fusion reaction and which is believed to contribute to the unexpected sulfdation resistance of the resultant coating. In addition to the unexpected sulfidation resistance, the coating of the present invention also possesses good high temperature ductility and the surface of the coating is relatively smooth.

A photomicrograph is shown in FIG. 3 taken of a protective coating surface at a magnification of 480 times which had been etched for seconds with Marbles reagent. The refractory aluminum oxide particles are indicated at 22. The boride compounds and complexes formed in situ during the exothermic reaction are not clearly visible although a portion of the particles are concentrated at the grain boundaries indicated at 24.

In accordance with an alternative method aspect of the present invention, a solid part or ingot of the cermet-type alloy can be prepared in accordance with the general arrangement illustrated in FIG. 4. As shown, a powder blend 26 of the desired composition can be placed in the cavity of a refractory mold 28 having a contour corresponding to the shape of a part to be fabricated therefrom. The mold is placed in a suitable furnace 30 provided with an inert protective atmosphere and the mold and powder blend are heated to an elevated temperature so as to initiate a fusion of the metallic powder, as well as the initiation of the exothermic reaction. Upon completion of the exothermic reaction and a fusion of the matrix metals, the mold is removed from the furnace and allowed to cool,

whereafter the solidified part is extracted from the mold. The solid cermet-type alloy component has a metallurgical structure similar to that illustrated in FIG. 3. It will be appreciated from the foregoing that any one of a variety of intricate shapes can be made of the cermet-type alloy employing the foregoing technique requiring only minimal machining operations to provide a final part possessing the required configuration and dimensional accuracy.

In order to provide a comparison of the sulfidation resistance of the cermet-type alloy comprising the present invention with the various well known high-temperature corrosion-resistant alloys and so-called superalloys, an accelerated suliidation test was devised. The test employs test specimens of a size 3 inches by 1 inch by 1&2 inch cut from cold-rolled Inconel 600 which is susceptible to suldation attack and therefore makes an ideal base metal for fast and reliable testing of various protective coatings. The corrosive medium comprises a mixture consisting of by weight of sodium sulfate and 10% of sodium chloride which is placed in an alumina Crucible and surrounds the lower portion of an Inconel test specimen having a protective coating thereon. The salt solution is heated to a temperature of 1650 F. in an air atmosphere furnace for a period of two hours and thereafter the coated test specimen is withdrawn from the molten salt bath to a position thereabove and is subjected to a further two-hour heating period at that temperature to complete one cycle of the oxidation phase. At the completion of the four-hour cycle, the test specimens are cooled and inspected for suliidation and corrosion attack.

Failure of the test specimens is evidenced by edge cracking along the plane of the face of the specimen, which is evidenced by a delamination due to grain boundary failure as a result of progressive migration of the suldation attack from the opposite faces of the test specimen. This test has been found particularly effective for evaluating sulfdation attack of jet engine components which are exposed to high temperature environments including sulfur from the burning of jet fuel and salt from sea atmospheres. The foregoing tests conducted on Inconel 600 specimens without any protective coating in comparison to similar specimens incorporating protective coatings of the cermet-type alloy of the present invention have revealed improvements in the suldation resistance of upwards of I800%.

The foregoing favorable test results have been substantiated by tests of these same materials under simulated jet engine operating conditions in which coated turbine blades were subjected to contact with hot jet fuel combustion gases in the presence of salt Water vapors.

In order to further illustrate the novel cermet-type alloy comprising the present invention, the following examples are provided. It will be understood that the examples as hereinafter set forth are provided for illustrative purposes and are not intended to be limiting of the scope of this invention as herein described and as set forth in the subjoined claims.

In the preparation of the particulated mixture as described in the following examples, prealloyed powders were employed in some instances according to the preferred practice of the present invention. In each instance, the refractory aluminum oxide constituent was introduced in a substantially pure form. By varying the proportions of the several powders, hereinafter designated as A, B and C, controlled variations are achieved in the particular composition of the suldation resistant cermet-type alloy produced. It will, of course, be appreciated that alternative powder mixtures can also be satisfactorily employed including elemental powders either alone or in combination with suitable prealloyed powders. Generally, the use of elemental powders of the titanium or zirconium constituents is undesirable since the reactive nature of these two metals, when in a 'finely particulated state, necessitates in many instances the use of inert atmospheres or, alternatively, high vacuums to avoid oxidation attack.

The compositions and characteristics of prealloyed powders A and B, as well as the refractory aluminum oxide powder C, are as follows:

POWDER A Composition Ingredient: Percent by wt. Titanium 70 'Nickel 30 Screen analysis Mesh: Percent by wt. +100 0.04 100+200 6.11 -200-i-325 17.68 -325 76.17

POWDER B Composition Ingredient: Percent by wt. Chromium 15.05 Boron 3.53 Carbon 0.03 Nickel Balance Screen analysis Mesh: Percent by wt. +120 vNil 120+150 4.1 -l50-I-200 24.4 -200-l-325 34.3 -325 37.0

POWDER C Composition Ingredient: Percent by wt. Aluminum oxide (-325 mesh) 100 EXAMPLE I A powder mixture was prepared comprising 500 grams of powder B, 60 grams of powder A and 42 grams of powder C, which corresponds to a stoichiometric proportion for the formation of the compound TiB2, with a slight excess of free Ti in the metallic matrix. The resulting powdered mixture was applied to the surface of a sheet of `Inconel 600 employing an organic binder comprised of an acrylic resin dissolved in a volatile solvent, and thereafter was heated for a period of 30 minutes at 2100o F. in a high-vacuum atmosphere to effect a fusion and exothermic reaction of the constituents thereof. The resultant protective coating formed was of a gray, smooth appearance and was observed to have exceilent resistance to suldation when heated for 12 hours at 1650 F. in a molten salt mixture, consisting of 90% Na2SO.,l plus 10% NaCl, with an additional heating in open air at 1650 F.

The resultant coating had a theoretical composition of:

Percent N1 70.5

Cr 12.5 TiB2 9.5 Ti 0.5 A1203 7.0

EXAMPLE II A powder mixture was prepared comprising 500 grams of powder lB, 60 grams of powder A and 30 grams of powder C, which corresponds to a stoichiometric cornposition with reduced amounts of Al203. The resulting powder mixture was applied to the surfaces of cold-rolled sheets of Inconel 600, Hastelloy X, vL-605, Waspalloy and 304 stainless steel. Subsequently, the coated base metals were heated for a period of 30 minutes at a temperature of 2100 F. in a medium high vacuum atmosphere to produce a well-fused smooth, silver-gray coating. Suldation tests were performed with all five base metals coated with the given coating, showing an excellent resistance to suldation of all coated base metals.

The resultant coating had a theoretical composition of Percent EXAMPLE III A powder mixture was prepared comprising 500 grams of powder B, 60 grams of powder A and 17 grams of powder C, which corresponds to a stoichiometric composition with a very small amount of A1203 addition. The resulting powder mixture Was applied to the surfaces of Inconel 600 strips. The coated Inconel 600 strips were heated for a period of minutes at 2100 P. in a medium high vacuum atmosphere to produce a well-fused smooth, silver-gray coating, which was of high resistance to sulidation.

The resultant coating had a theoretical composition of:

Ni Cr A powder mixture was prepared comprising 500 grams of powder B, 57 grams of powder A and 28 grams of powder C, which corresponds to a stoichiometric composition with a slight excess of boron and 5% of A1202 powder. The resulting powder mixture was applied to the surfaces of Waspalloy strips employing an organic binder, and thereafter was heated for a period of 30 minutes at 2100D F. in a high vacuum atmosphere to effect a fusion and exothermic reaction of the constituents thereof. The resultant protective coating formed was of a gray, smooth appearance and was observed to have a good resistance to sulfidation.

The resultant coating had a theoretical composition of:

Percent Ni 70.7 Cr 12.5 9.8

EXAMPLE V A powder mixture was prepared comprising 500 grams of powder B, 40.73 grams of pure Ti powder, and 40 grams of pure A1203 powder, which corresponds to a stoichiometric composition for formation of TiB2, without any excess of Ti or B, with an addition of about 5 wt. percent A1203. The resulting powdered mixture was applied to the surfaces of Inconel 600 and Hastelloy X strips employing an organic binder, and thereafter was heated for a period of 30 minutes at 2100 F. in a high vacuum atmosphere to effect a fusion and exothermic reaction of the constituents thereof. The resulting protective coating formed was of a silver-gray appearance and was observed to have resistance to suldation.

EXAMPLE V I Ingots having a nominal weight of about 100, grams were prepared employing powder mixtures of the same composition as previously described in Examples I-V. These ingots were prepared by filling a refractory crucible with the powder mixture and thereafter placing the crucible in a furnace provided with a substantially dry inert argon atmosphere. The powder mixtures were heated to a temperature of about 2100 F. for a period of about 30 minutes. In each case, yan ingot was obtained having a peripheral contour corresponding to the contour of the Crucible employed and characterized as having a continuous phase of the matrix metal through which particles of the titanium diboride reaction compound and the aluminum oxide were uniformly distributed.

EXAMPLE VII Protective coating are prepared in the same manner as hereinbefore described in connection with Examples I-V employing the same particulated mixtures, to which about 6% of titanium nitride (TiN) is added.

In addition to the foregoing, satisfactory coatings and ingots of the cermet-type alloy comprising the present invention are prepared employing powder compositions which are processed in a manner similar to that described in Examples I-VII to produce reacted alloys to the following compositions.

EXAMPLE VIII Ingredient: Percent by wt. Ni 70 Cl' L 25 TiBz 3 Al203 2 100 EXAMPLE IX Ingredient: Percent by wt. Co 55 Ni 22 TiBz 9 A1203 4 100 EXAMPLE X Ingredient Percent -by wt. Co 30 ZI'BZ A1203 5 100 EXAMPLE XI Ingredient Percent by wt. Ni 17 Co 5 Cr 30 zrB, 4o A103 8 100 EXAMPLE XII Ingredient: Percent by wt. Ni 15 Co 15 Cr 40 TiB2 25 A1203 5 100 EXAMPLE XIII Ingredient: Percent by wt.

`Co 85 Cr zrB2 3.5 A1203 1.5

EXAMPLE IX Ingredient: Percent by wt. Ni 40 Cr 30 TiB2 10 zrls2 10 A1203 5 While it will be apparent that the invention herein disclosed is well calculated to achieve the benefits and advantages as hereinabove set forth, it Will appreciated that that invention is susceptible to modification, variation and change without departing from the spirit thereof.

What is claimed is:

1. The method of forming a cermet-type alloy which comprises the steps of forming a particulated mixture comprising matrix metals, reactive constituents and refractory aluminum oxide particles; said matrix metals present in an amount of from about 50% to about 97% of said mixture and consisting essentially of about 10% to about 60% chromium and the balance comprising at least one metal selected from the group consisting of nickel and cobalt, said reactive constituents comprising boron and at least one reaction metal selected from the group consisting of titanium and zirconium present in an amount sufficient upon reaction thereof to produce from about 2% to about 40% of the corresponding metal boride, said refractory aluminum oxide particles present in an amount of from about 1% up to about `8%, heating said mixture to an elevated temperature in a substantially inert atmosphere for a period of time sufficient to effect at least a partial fusion of said mixture and an exothermic reaction 4between said reactive constituents whereupon the corresponding metal boride and said refractory aluminum oxide particles are dispersed as discrete discontinuous phases throughout a continuous phase of said matrix metals.

2. The method as defined in claim 1, wherein said matrix metals are present in an amount of from about 60% to about 90%, said reactive constituents are present in an amount sufficient upon reaction thereof to produce from about 4% to about 30% of the corresponding metal boride and said refractory aluminum oxide particles are present in an amount of from about 5% to about 7%.

3. The method as defined in claim 1, wherein said chromium is present in an amount of from about 15% to about 35%.

4. The method as defined in claim 1, in which the particles of said particulated mixture are of a size ranging from about 2O to about 500 microns.

5. The method as defined in claim 1, wherein said reactive constituents and said matrix metals are in the form of prealloyed powder particles.

6. The method as defined in claim 5, wherein at least some of said prealloyed powder particles are of a composition corresponding to the low melting point eutectic of the constituents thereof.

7. The method as defined in claim 1, including the further step of confining said particulated mixture in a mold of a preselected configuration such that the resultant fused mass of said alloy is of a configuration conforming with the configuration of said mold.

8. The method as defined in claim 1, in which said reactive constituents comprise boron and titanium present in stoichiometric proportions to form the corresponding titanium diboride reaction product.

9. A metal article having a dense non-porous cermettype sulfidation resistant protective coating on at least a portion of the surface thereof, said coating comprising about 50% to about 97% of a continuous metallic matrix consisting essentially of from about 10% to about 60% chromium and the balance comprising at least one metal 'selected from the group consisting of nickel and cobalt,

said metallic matrix having interspersed therethrough discrete discontinuous phases in compressed condition comprising refractory aluminum oxide particles and a ceramic compound selected from the group consisting of titanium borides, zirconium borides and mixtures thereof; said refractory aluminum oxide particles being present in an amount of about 1% to about 8% and said ceramic compound being present in an amount of from about 2% to about 40%, said coating t'enaciously bonded to and alloyed with said article at the interface therebetween.

10. The metal article as defined in claim 9, in which said coating comprises about 60% to about 90% of said metallic matrix and wherein said refractory aluminum oxide particles are present in an amount of about 5% to about 7% and said ceramic compound is present in an amount of about 4% to about 30%.

11. 'Ihe metal article as defined in claim 9, in which said metallic matrix contains about to about 35% chromium.

12. The metal article as defined in claim 9, wherein said ceramic compound consists essentially of titanium diboride and complexes thereof.

13. A cermet-type alloy comprising from about 50% to about 97% of a dense, non-porous continuous metallic matrix consisting essentially of about 10% to about 60% chromium, and the balance comprising at least one metal 'selected from the group consisting of nickel and cobalt, said continuous metallic matrix having interspersed therethrough in a compressed condition discrete discontinuous phases comprising refractory aluminum oxide particles and ceramic compounds consisting essentially of titanium borides, zirconium borides aud mixtures thereof; said ceramic compound being present in an amount of about 14 2% to about 40% of said alloy and said refractory aluminum oxide particles being present in an amount of about 1% to about 8% of said alloy.

14. The alloy as defined in claim 13 in which said metallic matrix comprises about to about 90% of said alloy and said ceramic compounds are present in an amount of about 4% to about 30% and said refractory aluminum oxide particles are present in an amount of from about 5% to about 7%.

15. The alloy as deiined in claim 13, in which 'said chromium is present in an amount of about 15% to about 35%.

16. The alloy as defined in claim 13, in which said refractory aluminum oxide particles are of a size less than about 2O microns.

References Cited UNITED STATES PATENTS 2,775,531 12/1956 Montgomery et al. 29-195 X 2,903,375 9/1959` Peras 29-195 X 2,994,124 8/1961 Denny et :al 29-195 X 3,084,064 4/ 1963 Cowden et al 29-195 X 3,091,548 5/1963 Dillon 29-195 X 3,110,571 11/1963 Alexander 29-195 3,215,511 11/1965 Chisholm et al 29'-195 X 3,547,673 12/ 1970 Bredzs et al. 29-l95 X L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X..R. 

